It's not possible to merge conflicts in the same branch.

[MoezGholami]

When there is a conflict in the one branch only (pulling conflicting commits), this plugin does not work. I will get Conflict markers miss common base revision. Ensure you're using 'merge.conflictStyle=diff3' in your gitconfig in my footer.
I've already created a Git conflict simulation for my students which simulates a conflict. It can be used to further improve this plugin.
Here is the screenshot of my vim after trying the mergetool.

[idbrii]

Are you using merge.conflictStyle=diff3? I don't see diff3 anywhere in the repo you linked.

See here: https://github.com/samoshkin/vim-mergetool#conflict-markers-format